The present invention relates to a method and apparatus for measuring the amount of entrained gases in a liquid sample.
Entrained gases are usually mentioned in the context of papermaking processes involving a flow of stock, and typically include free air and bound, residual or stabilized air. The stock flow usually includes a slurry of fibers, charged micro-particles such as talc, magnesium carbonate or calcium carbonate, coagulants and/or flocculants such as charged polymers, and starches, all of which are carried in a liquid medium such as water. Turbulent flows in open channels, leaking pumps, and the free-fall of stock or additives into a chest are among the conditions that contribute to the entrainment of air in papermaking stock. Intense agitation of the stock generates shear forces, which can subdivide incoming air bubbles and attach them to components of the stock. The free air portion or unstabilized portion of the entrained gases consists of freely moving air bubbles or free bubbles in the stock, which typically have an individual bubble diameter larger than 70 to 100 xcexcm. The stabilized air portion of the entrained gases in turn includes air bubbles that are small enough to adhere to fibers, or to be found inside fibers. The average diameter of the bubbles making up the stabilized air portion is typically less than 70 to 100 xcexcm, with the most common diameter distribution being from about 30 xcexcm to about 170 xcexcm. In addition to entrained gases, the stock may also contain dissolved gases. Chemical or biological activity may also produce dissolved gases directly in the stock. Dissolved gases are in the form of molecules in the liquid. The equilibrium between the entrained gases and the dissolved gases depends, among other things, on the prevailing temperature and pressure, and conductivity.
The measurement of entrained gases in liquids is useful in various applications, such as, for example, in the papermaking industry. General experience has shown that the adverse effects of entrained gases on papermaking stock and final sheet production include, among other things, the generation of foam, spots on paper sheets, pin holes, retarded drainage, energy losses in pumps, and sheet breaks. Gas bubbles trapped inside formed paper sheets decrease the number of fiber bonds, decrease the formation of fiber bundles, and decrease the build-up of fiber bundles. Additionally, dissolved gases in paper making stocks tend to maintain aerobic slime and bacteria growth, causing not only quality problems but also more frequent boil-outs and increased consumption of boil-out compounds and cleaning agents. Therefore, an accurate measurement of the actual amount of entrained gases in a papermaking stock is desirable in preventing the above-stated negative effects.
Devices and methods for measuring the amount of entrained gases in a liquid sample are known. According to one method, the Coriolis force necessary to maintain a resonant frequency of a fluid-filled tube is measured and the force or resonant frequency required is used as an indicator of density. This method aims at measuring density to arrive at a determination of the amount of entrained gases in the liquid sample being considered. By way of example, a vibrating U-tube densitometer has recently been proposed for the measurement of entrained gas bubbles. A U-tube is an example of a device that measures resonant frequency to determine density. According to another method called the xe2x80x9cBroadwayxe2x80x9d or xe2x80x9cexpansionxe2x80x9d method, a sample is taken of a liquid and a fixed volume thereof is isolated in a sample chamber under atmospheric pressure. A sudden vacuum is then applied to the sample, which results in an expansion of the entrained gases. The increased volume of the sample is used to measure the amount of entrained gases and dissolved gases in the sample.
According to yet another method, a liquid sample is taken under pressure, such as with a mechanical screw or piston. The decrease in the volume of the sample owing to the pressure increase is measured, and the volume of entrained gases is determined from the measured decrease in volume. The rotation rate of the mixing pump in the papermaking system has also been used to estimate changes in the content of entrained gases, as these parameters correlate relatively well.
A further method involves determining a weight difference of a liquid sample. A liquid sample containing entrained gases is weighed on a scale. The same kind of sample with de-gased liquid is also weighed. The de-gased stock may be obtained by boiling the stock under vacuum to remove any gases therefrom. The gas content is then determined from the weight difference.
Disadvantages of the above existing methods for measuring the amount of entrained gases in a liquid sample include, among other things, an inability to obtain accurate measurements of the actual amount of entrained gases in a liquid sample where the sample is taken from a dynamic liquid system, that is, from a liquid system where the liquid is flowing and where the amount of entrained gases in the liquid may change as a function of time. Typical of a dynamic liquid system would be a paper stock processing system that includes a flow of stock having water as the carrier medium. In such a system, the air content in the flowing liquid is not constant, and varies throughout the process. Random changes in air volume in a paper stock processing system have been measured as being typically between 5 and 20%, but could be as much as 200%, with changes in air volume having the capacity to be either fast or slow. In such a system, existing methods for measuring the amount of entrained gases would be disadvantageous, in part because the method is based on an isolated, stationary stock sample, the quality of the results therefore depending largely on how well the sample represents the average stock at any given time. Additionally, any stationary liquid sample could lead to an escape of entrained gases after even a short period of time, therefore further altering any measurements of entrained gases within that sample.
One method of obtaining measurements of the actual amount of entrained gases in a liquid sample involves the use of on-line sensors. Examples of such sensors include ultrasonic sensors and sensors for sensing sample compressibility.
The principle of operation of ultrasonic sensors is based on the fact that ultrasonic energy will attenuate in a liquid containing entrained gas bubbles, with the attenuation being a function of the amount of gas bubbles. According to this method, a liquid sample is fed from a dynamic liquid system, such as a paper stock processing system, through an ultrasonic sensor for measuring the amount of entrained gas bubbles in the sample. However, although the attenuation of ultrasound is very sensitive to the quantities of stabilized air bubbles present, it is insensitive to free bubbles. In addition, the attenuation efficiency of the air bubble measurement is typically a function of the papermaking system within which the stock flow occurs because the efficiency depends on the distribution of the bubble size and on the mechanism of stabilization of the air bubbles. While the quantity of gaseous air is linearly correlated with the attenuation of ultrasound when the air content is below about 0.9% by volume, if the air content is larger, the number of large, free bubbles increases and the dependence of attenuation on air content is no longer valid.
In addition, on-line sensors are known which make a measurement of entrained gases by compressibility of the process liquid. An example of such a sensor is the PULSE AIR(trademark) sensor, provided by Product and Process Engineering Concepts. The PULSE AIR(trademark) sensor is inserted directly into the process liquid, and determines the amount of entrained gases therein by measuring fluid compressibility. Unfortunately, it is not a continuous measuring system.
The present invention provides a method for measuring the amount of entrained gases in a liquid sample. The method comprises the steps of providing a conduit defining a volume, the conduit having an inlet at a lower end thereof and an outlet at an upper end thereof; flowing liquid sample into the inlet, through the conduit, and out of the outlet; and determining the weight of liquid sample in the conduit during a flow of liquid sample through the conduit to obtain a first measurement. Thereafter, a difference between the first measurement and a predetermined weight value representing a weight of the same liquid sample flowing through the conduit but containing no entrained gases is determined. The determined difference is then used to obtain a calculated weight or volume of entrained gases in the liquid sample flowing through the conduit.
The present invention further provides an apparatus for measuring the amount of entrained gases in a liquid sample. The apparatus comprises a conduit defining a volume and comprising an inlet at a lower end thereof and an outlet at an upper end thereof; and a weight measuring device coupled to the conduit for measuring a weight of liquid sample flowing through the conduit from the inlet thereof to the outlet thereof to obtain a first measurement. The apparatus further includes a control unit coupled to the weight measuring device. The control unit determines a difference between the first measurement and a predetermined weight value representing a weight of the same liquid sample flowing through the conduit but containing no entrained gases. The difference is used to obtain a calculated weight or volume of entrained gases in the liquid sample flowing through the conduit.
In addition, the present invention provides a system comprising: the apparatus of the present invention as described above; a flow of liquid in a main processing circuit of a processing system; and a diverting device for diverting at least a portion of the flow of liquid in the main processing circuit as a liquid sample through the apparatus.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are only intended to provide a further explanation of the present invention, as claimed. The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate several exemplary embodiments of the present invention and together with description, serve to explain the principles of the present invention.